Thermal Poling Assisted Ion Exchange in LAS Glass and Glass-Ceramic

An open electrode thermal poling setup was used to introduce Na+ and K+ cations from chloride salt coatings into the surface of LAS glass. The thermal poling was performed at 200 °C for 180 min and 540 min at poling currents of 50 µA and 200 µA. Na+ and K+ migrate from the salt coatings along the electric field lines into the glass surfaces up to 14 µm and 4 µm in depth, respectively. As these cations are larger than the contained Li+-ions, the induced stresses lead to an increase of surface hardness by ca. 15 % in the ion exchanged areas. Etching trials with 5 % HF solution revealed a faster etching of these Na+ and K+ enriched surface areas. The ceramisation of the treated samples led to uneven surfaces presumably due to the influence of Na+ and K+ on local viscosity in combination with altered crystallisation behaviour. The introduced cations accumulate in the valleys of the surface topology in a 1-2 µm thin glassy surface layer. Contrary to the glass samples, these areas etch away more slowly in the etching trials and reveal protruded domains where the salt crystal contact areas (abbreviated as cca) were in first place. The high quartz solid solution crystal content in the surface is highest for NaCl coated samples followed by KCl coated samples and uncoated but thermally poled samples.